Water Flow Rate (GPM) Calculator
Calculate water flow rate in GPM, L/min, and m3/hr from pipe diameter and system pressure using the Torricelli-based flow equation.
Results
What is it?
Water flow rate (GPM) through a pipe depends on the pipe diameter and the pressure driving the flow. This calculator uses a simplified Torricelli-based formula that estimates flow rate from pressure and cross-sectional area. In practice, actual flow rate is also affected by pipe length, fittings, and pipe roughness (friction losses), so this gives a theoretical maximum at the outlet.
How to use
1. Enter the pipe inner diameter (ID) in inches. Common sizes: 0.5" (kitchen tap), 0.75" (main supply), 1" (irrigation), 1.5" (larger irrigation). 2. Enter the static water pressure in PSI (use a pressure gauge on a hose bib; typical residential = 40-80 PSI). 3. The calculator returns flow rate in GPM, L/min, and m3/hr.
Example scenario
A 1-inch irrigation pipe at 60 PSI. Pipe area = pi x 0.5^2 = 0.785 in2. Using this calculator, flow rate is approximately 4-6 GPM depending on friction losses. Typical drip irrigation emitters use 0.5-1 GPM each, so this supply line can feed 4-12 emitters simultaneously.
Pro tip
This formula gives theoretical flow at the pipe exit with no friction losses. Real-world flow through long pipe runs is significantly lower — use the Hazen-Williams formula for accurate friction loss calculations in distribution systems. As a rule of thumb, keep flow velocity below 5 ft/s in supply piping to avoid noise and water hammer.